Data synthesis
Continuous exercise exposure
In this category, 12 studies, 1 controlled intervention,47 4 case–control studies,48–51 5 case reports25 43 44 52 53 and 2 observational studies,54 55 including a total of 251 elite or competitive amateur female athletes of various types of sports, such as endurance, technical, weight class, aesthetic and ball sports, were identified. Typically, the training duration was more than 10 hours per week, with a maximum specified training duration of 14 hours per week on average during pregnancy.25 The study of Kardel et al reported a training duration of 8.4 hours/week in the high-exercise group (HEG). This is the only study to date that has investigated higher intensity training (heart rate between 170 and 180 beats/min during interval training) during pregnancy, so it was decided to include this study as well.47
Fertility
The average duration between becoming pregnant and ending contraception was 1.7 months (from 0 to 7 months) in the study of Penttinen et al.49 Contraceptive pills were used by 13 athletes for a mean of 4.5 years before their first pregnancy, and 1 athlete used an intrauterine device for 2 years.49 Sundgot-Borgen et al also observed no differences in fertility problems between 34 elite athletes and 34 active controls.51
Pregnancy disorders, complications and injuries
In three studies, pregnancy was described as uncomplicated.43 51 53 Beilock et al stated the following maternal complications, once each: sinus problems, heavy bleeding, hypertonia and kidney stones.54 Further physical complaints, such as upper respiratory tract infections, pelvic and back pain, nausea and fatigue, were reported, and one athlete presented with pneumonia, laryngitis and cholestasis.44 47 48 In the case–control study of Bo et al, low back pain, pelvic girdle pain and urinary or faecal incontinence during pregnancy and after childbirth were noticed, but no significant differences in the prevalence between the elite athletes and the control group could be observed. The rate of urinary stress incontinence symptoms at 6 weeks postpartum was 29% (n=9) in elite athletes and 30.4% (n=14) in the control group, and at the completion of the questionnaire, it was 35.5% (n=11) in elite athletes and 26.1% (n=26.1%) in the control group.48 It was stated that four athletes experienced training-induced injuries in the third trimester, and four athletes (two endurance-type sports; two team ball sports) suffered from a stress fracture postpartum.51 Two sacral fractures (13–18 weeks and 19–24 weeks postpartum) after rapid resumption of postpartum sports activities were described.25 Fifty per cent (n=21) of the athletes experienced an injury postpartum, of which 6 were bone stress injuries, 11 were musculoskeletal injuries (muscle, tendon and/or ligament strain/sprain or rupture), 2 were sciatic problems and 2 were not further specified.56
Miscarriage
Seven athletes (23%) experienced an abortion during their first trimester in a previous pregnancy.49 Sundgot-Borgen et al documented no differences in miscarriage between 34 elite athletes and 34 active controls (three miscarriages in 28 elite athletes (11%) and eight miscarriages in 29 inactive controls (28%), where three controls had two miscarriages).51
Preterm/post-term birth
No differences regarding preterm birth could be found in the case–control studies from Bo et al, Penttinen et al and Sundgot-Borgen et al.48 49 51 Based on the information about training provided, a birth at term can be assumed for another study.25 The athlete monitored in the case report of Bubnjević et al gave birth at 39 weeks+4 days.53 The twins were born after 36 weeks.44 Only one post-term birth (11 days) was reported.43 The average gestational age at delivery was 39.0±1.6 weeks in the study of Darroch et al, and no detailed information on preterm or post-term births was provided.56
Labour
Three preterm labours in the study of Beilock et al were observed.54 Kardel et al detected a significantly later onset of labour (mean difference of 1.2 weeks) in the medium-exercise group (MEG) than in the HEG, but only for mothers giving birth to girls.47
No significant differences could be found in the length of labour between the groups (6.7±5.5 hours in the MEG vs 9.4±5.9 hours in the HEG)47; first stage of labour in actives 626±332 min versus 576±322 min in controls and second stage of labour in actives 27.9±17.8 min versus 27.8±27.2 min versus in controls49; first stage of labour non-athletic 603 min (range 231–1069), high-impact 600 min (range 296–1386), low-impact group 613 min (range 331–1017), second stage of labour non-athletic 57 min (range 17–116), high-impact 56 min (range 32–106), low-impact group 65 min (range 23–153).50 Beilock et al confirmed one extended labour, providing no further details.54 Kardel et al reported that the duration of labour tended to be shorter in the MEG than in the HEG (normal delivery 6.7±5.5 vs 9.4±5.9 hours; instrumental delivery 8.0±2.6 vs 11.7±3.4 hours), although this was not statistically significant.47
No significant distinctions in the birth mode between the groups (vaginal, vacuum extraction, forceps or caesarean section) were stated.48 49 Other authors observed no significant differences with regard to the incidence of emergency caesarean section.50 The twins were delivered by elective caesarean section.44 Darroch et al reported 30 vaginal births and 11 caesarean sections.56 In the case report of Almquist et al, a natural birth without complications was stated.52
The rate of vaginal or perineal tears did not differ significantly (athletes 0 vs controls 0) in the study of Penttinen et al.49 Compared with the high-impact athletes, among the low-impact athletes, a significantly higher incidence of third-degree and fourth-degree perineal tears was detected, whereas no significant differences were seen in the comparison between the control group and the athletes (both low and high impact).50 One labour was induced due to a reduced amount of amniotic fluid, which was observed in the last sonography in the 39th week.53
Birth weight
Several authors revealed no significant differences in birth weight between the athletes and controls (elite athletes 3290.6±751 g vs controls 3466.4±529 g,48 MEG 3590.5±532 g vs HEG 3650.7±515.8 g,47 athletes 3460±484 g vs controls 3335±503 g,49 athletes 3607.1±544.7 g vs controls 3587.3±610.1 g).51 In one case report, the birth weight of twins was 2200 g and 2300 g (after a 36-week gestation period)44; in another, it was 3060 g (39 4/7 weeks).53 In the case report of Almquist et al, birth weight was 3238 g at 40 weeks of pregnancy.52 Beilock et al reported two children with low birth weight.54
APGAR score/infant development
The following information was provided on the health status of the children: during the twin pregnancy, fetal development was stated as normal, and after birth, the twins were in good health and showed normal development 12 months after birth.44 Likewise, Bung et al and Solli et al described athletes who gave birth to healthy infants.25 43 One study stated ‘unhealthy’ as a fetal complication during pregnancy, and further information was not provided.54 There were no reports of abnormal APGAR scores.43 47 51 53 57
Exercise behaviour during pregnancy and postpartum
The majority of the studies in this group confirmed a reduction in training volume and intensity during pregnancy.25 44 50 51 53 54 56 In the study of Beilock et al, women decreased cardiovascular and resistance exercise by 49% in the first trimester, by 72% in the second trimester and by 80% in the third trimester.54 Two case reports described a reduction in running volume from prior to pregnancy to the ninth month of pregnancy from 100 to 20 km/week,53 and from 155 to 72 km/week.44 Solli et al detected in a case report an increased training volume from a median of 10 hours/week in gestational weeks 1–12 to 18 hours/week in gestational weeks 13–28 but a decline from weeks 13–28 to weeks 29–40 (median 18 to 9.6 hours/week).25 Sundgot-Borgen et al revealed a decent strength and endurance training volume from prepregnancy to the first and third trimesters (p<0.001) and 0–3 months postpartum (p<0.001), but the exact values were not provided.51 Sigurdardottir et al differentiated only between before and during pregnancy and presented the following values: in the low-impact group 20.3±10.7 hours/week vs 10.2±12.7 hours/week and in the high-impact group 14.3±4.3 hours/week vs 8.5±7.4 hours/week.50 Darroch et al reported a significant reduction in running volume from the first (63±34 km/week) to the third trimester (30±30 km/week).56 Penttinen et al reported that 23 athletes (77%) continued their training until week 23 of gestation, and 18 (60%) still competed to week 13 of gestation.49 Postpartum, 18 athletes continued to compete at a median interval of 8.2 months (range 2–24 months).49 While 2 athletes achieved better condition than before pregnancy, 11 reached the same level of performance and 5 did not.49 Bo et al stated that 38% of athletes started running within 6 weeks postpartum. Sundgot-Borgen et al reported that most athletes (71%, n=24) returned to their training within 0–6 weeks, 24% (n=8) within 7–12 weeks and 6% (n=2) within 13–18 weeks postpartum.51 After delivery, in the case report of Bung et al, the athlete was able to resume her normal training within a few weeks, and after less than 6 months, she was able to beat her personal best time in various short distances.43 Another paper revealed that the mother returned to competitive racing 8.5 weeks after her delivery.44 In the study of Darroch et al, athletes returned to activity/exercise after approximately 6 weeks, and after 3 months, they reached 80% of their prepregnancy trainings volume.56
Acute exposure to intensive activity during performance testing
Three papers evaluated performance testing during pregnancy in 21 highly active women and elite athletes.57–59 Another two case reports of ‘continuous exercise exposure’ also mentioned performance testing during pregnancy.43 44
Salvesen et al evaluated performance testing at 23–29 weeks of pregnancy,58 and Szymanski et al evaluated performance testing at the 29th and 33rd weeks of gestation.59 In another paper, Szymanski et al described two treadmill sessions (moderate and vigorous intensity) in the same population, where women from the exercise group (regular and highly active) underwent an additional vigorous-intensity session (30-min treadmill) with a target heart rate of 60%–84% heart rate reserve.57 A case report described bicycle ergometer testing and track running from the 28th week of pregnancy every 2 weeks, during puerperium, 6 weeks and 6 months postpartum.43 Davies et al reported weekly standardised submaximal field running tests during the first 32 weeks of pregnancy as well as standardised submaximal treadmill tests 29 weeks antepartum and 10 weeks postpartum.44 The effect of the performance testing on different outcome parameters will be described in the following.
Umbilical and uterine blood flow/fetal heart rate
Uterine and umbilical arteries monitored by Doppler ultrasound showed a reduction in the mean uterine artery volume blood flow to 40%–75% of the initial value during testing.58 Fetal bradycardia (an unfavourable outcome indicator) and a high umbilical artery pulsatility index (PI) were detected, and the mean uterine artery volume blood flow was less than 50% of the initial value.58 In a highly active exercise group (n=15), five women (33.3%) experienced a deceleration of fetal heart rate (fHR) postexercise (lasting between minutes 2:08 and 3:12, pre-exercise 142.8±10.4 vs postexercise 81.8±10.4) and alterations in umbilical and uterine artery Doppler indices.59 In the same population, but after an additional vigorous-exercise-intensity session, Doppler indices (umbilical artery Doppler S/D ratio, resistance index, pulsatile index) decreased significantly (umbilical PI pre-exercise 0.85±0.07 vs postexercise 0.82±0.09).57 However, no significant differences in any umbilical artery Doppler indices were observed pre-exercise and postexercise with moderate exercise.57 Salvesen reported that both fHR and umbilical artery PI normalised quickly after exercise termination (figure 4).58 In the case report of Bung et al, the maternal heart rate reached values of over 170 beats per minute (bpm) during sprints.43 The fHR measured immediately after exercise indicated bradycardia (70 bpm), which recovered to 120 bpm within 3 min, and the athlete had dizziness and precollapse-like symptoms.43
Figure 4These data were extracted from a prior original study58 and are from six pregnant women before, during and after exercise. After a warm-up for 10 min (maternal heart rate (mHR) of ~135/min), the women ran three to five submaximal workloads (test 1 to test 5; 5 min each) on a treadmill with a VO2max (maximal oxygen consumption) of 60%–90%. Fetal heart rate (fHR) in beats per minute (data points in blue) and umbilical artery pulsatility index (PI) (data points in green) were measured before exercise, after warm-up, during (test 1 to test 5) and after exercise. Three women completed only three workloads, one woman completed four workloads and one woman completed even five workloads. An increase in PI and a drop in fHR could be observed in the two women (woman 5 and woman 6) who completed four or five workloads each. Woman 5 exercised at 95% of maximal mHR and woman 6 exercised at 92% maximal mHR. These results imply that exercise intensity over 90% of maximal mHR may have a negative impact on fetal well-being.
Pregnancy disorders, complications and injuries
One woman developed HELLP syndrome at 35 weeks.58 No other complications or injuries were reported.43 57–59
Preterm/post-term birth
The following durations of pregnancy were reported for six pregnant women: 35, 36, 39, 40, 41, 42.58 Szymanski et al recorded two preterm births (one in the highly active group and one in the non-exerciser group at 36 1/7 weeks each).57
Labour
Four normal deliveries (66.7%), one caesarean section (16.7%) and one vacuum delivery (16.7%) were reported,58 the last two due to obstructed labour.58 In the woman with HELLP syndrome, labour was induced, and the woman delivered vaginally.58
Birth weight
The baby of the woman with HELLP syndrome had a birth weight of 2285 g (35 weeks of pregnancy).58 The other babies weighed a mean of 3252±177.8 g.58 The birth weight relative to gestational age was in the lower normal range (86%–103%) for Norwegian children. Szymanski et al detected no significant differences in fetal birth weight between the groups (non-exercisers 3460±427 g, regularly active 3.408±426 g, highly active 3.167±299 g).57 One woman in the highly active group delivered a small-for-gestational age baby (2690 g at 39 3/7 weeks), whereas two women delivered large-for-gestational age babies (one woman in the regular active group (4700 g at 39 5/7 weeks) and one in the non-exerciser group (4451 g at 41 0/7 weeks)).57
AGPAR score/infant development
No significant differences in APGAR scores were reported.57 A healthy girl was born to a woman with HELLP syndrome.58
High-risk sports
Only one study with female sport climbers met the inclusion criteria for this category.60
Preterm birth
Preterm birth within the 36th week was reported in 2 out of 15 women (13.3%).60 The risk of preterm birth was stated not to be higher than in the general population in Germany.60
Pregnancy disorders, complications and injuries
Two women experienced preterm births (36th week of gestation) but reported no complications and suffered no injuries or accidents while climbing.60
APGAR score/infant development
Fifteen women provided information about the delivery and the newborn; they reported that the children were healthy at birth and throughout the first 12 months postpartum.60
Exercise behaviour during pregnancy
Fifty per cent of the women in this study climbed until the 36th week of gestation, and all of them reduced their level of difficulty in training voluntarily by shifting to safer training options (more top rope).60
Rectus diastasis
None of the included studies in all three categories stated any effects on maternal diastasis of the rectus abdominus muscle.